Sputtering targets and method for the preparation thereof

ABSTRACT

A process for the preparation of a sputtering target which comprises sub-stoichiometric titanium dioxide, TiO x , where x is below 2, having an electrical resistivity of less than 0.5 ohm.cm, optionally together with niobium oxide, which process comprises plasma spraying titanium dioxide, TiO 2 , optionally together with niobium oxide, onto a target base in an atmosphere which is oxygen deficient and which does not contain oxygen-containing compounds, the target vase being coated with TiO x  which is solidified by cooling under conditions which prevent the sub-stiochiometric titanium dioxide from combining with oxygen.

The present invention relates to a process for the preparation ofimproved high rate sputtering targets and, in particular, to a processfor the preparation of sputtering targets comprising sub-stoichiometrictitanium dioxide with high electrical conductivity to be used in D.C.sputtering at high power levels.

Sputtered coatings of various oxides (e.g. silica) and nitrides (e.g.silicon nitride) are used to form optical coatings showing interestingproperties on a number of substrates. Known applications include lowemissivity films on window glasses, cold mirrors on reflectors, enhancedmirrors for photocopiers and antireflective coatings on picture glass orTV screens. These coatings are usually made of stacks of severaldifferent layers with different refractive indices, preferably acombination of low and high refractive index, to produce opticalfilters. For antireflective coatings it is preferred to combine twomaterials showing the highest and the lowest possible refractiveindices. Such materials are titania and silica. Another advantage ofthese materials is their durability. For low emissivity films on windowglasses it is preferred to combine a silver layer with a high refractiveindex material to dereflect the silver which improves lighttransmission.

Titanium dioxide coatings have a high refractive index and can thus beused to provide coatings of a high refractive index or to provide thehigh refractive index coatings in optical stacks. The existing processfor producing titanium dioxide coatings comprises using titanium metalas the sputtering target and using oxygen as a component of the plasmagas. The titanium is thus converted to titanium dioxide during thesputtering process. Although satisfactory coatings of titanium dioxidecan be produced, the rate of deposition is very slow and much slowerthan coating with zinc oxide and/or tin oxide.

As a substitute for titanium dioxide it has been suggested to usealternative materials such as niobium oxide. Whilst it is possible tocoat a substrate with niobium oxide using a niobium metal target atslightly higher speeds than the equivalent process using titanium,niobium is very expensive.

JP-A-07-233469 describes the preparation of a sputtering target byhot-pressing titanium dioxide powder in a nonoxidizing atmosphere andsintering the compact. The sintered compact comprises TiO_(x) where1<x<2 with a resistivity of 10 ohm.cm which is too high for D.C.sputtering at high power levels. The stability of the sputtering processand the arc rate are both very dependent upon the conductivity of thetarget, particularly at high power levels.

JP-A-62-161945 describes a method of manufacturing a ceramic sputteringtarget in which a ceramic material consisting mainly of ZrO₂, TiO₂,SiO₂, Ta₂O₃, Al₂O₃, Fe₂O₃ or a compound of these materials is sprayedusing a water plasma spray to produce a formed body which may be used asa sputtering target. The sputtering target is used for high frequencysputtering of non-conductive target materials.

Accordingly, there is a need for an improved process for the productionof sputtering targets comprising sub-stoichiometric TiO₂ which does notinvolve the hot-pressing and sintering route of JP-A-07-233469 and whichcan be used to produce such targets which have a high enough electricalconductivity to be used as large size targets with complex shapes athigh power levels.

We have now surprisingly discovered that titanium dioxide can be D.C.sputtered at high power levels from a target comprisingsub-stoichiometric titanium dioxide to provide a coating on a substrateof sub-stoichiometric or stoichiometric titanium dioxide.

Accordingly, the present invention provides a process for thepreparation of a sputtering target which comprises sub-stoichiometrictitanium dioxide, TiO_(x), where x is below 2 having an electricalresistivity of less than 0.5 ohm.cm, optionally together with niobiumoxide, which process comprises plasma spraying titanium dioxide, TiO₂,optionally together with niobium oxide, onto a target base in anatmosphere which is oxygen deficient and which does not containoxygen-containing compounds, the target base being coated with TiO_(x)which is solidified by cooling under conditions which prevent thesub-stoichiometric titanium dioxide from combining with oxygen.

Sub-stoichiometric titanium dioxide, TiO_(x), where x is below 2 andgenerally is in the range of from 1.55 to 1.95 is known in the art. Whenproduced according to the process of the present invention theelectrical conductivity will vary, depending upon the stoichiometry, themost preferred form having an electrical resistivity of 0.02 ohm.cm.

In carrying out the process of the present invention titanium dioxide,TiO₂ is plasma sprayed onto a target base, such as a backing tube orplate, for example a target base of an electrically conductive material,for example stainless steel or titanium metal, aluminium or copper. Thetarget may be of any type known in the art, for example a rotatabletarget or a flat magnetron target.

During the plasma spraying process, the action of the plasma flame onthe titanium dioxide causes the titanium dioxide to lose some oxygenatoms from its lattice, preferably from the surface of the particles.The titanium dioxide is converted into the sub-stoichiometric form, i.e.non-stoichiometric oxygen deficient titania. The primary plasma gas usedfor the plasma spraying is preferably argon, with hydrogen as thesecondary plasma gas in order to obtain the highest temperatures of theparticles. The titanium dioxide which is subjected to plasma sprayingpreferably has a particle size in the range of from 1 to 60 micrometres,preferably in the range of from 1 to 20 micrometres. Thesub-stoichiometric titanium dioxide which is coated on the target baseis solidified under conditions which prevent it from regaining oxygenand reconverting to TiO₂. Preferably the target base is water cooledduring the plasma spraying in order to quench the titanium dioxide inthe sub-stoichiometric form and to improve the conductivity thereof. Itis also important to use a certain amount of hydrogen or nitrogen in theplasma gas in order to produce a high temperature plasma and to assistin the reduction. Hydrogen is preferred because of its reducing powers.Preferably particle temperatures of above 2000° C. are used, morepreferably above 2500° C.

In a particular embodiment of the present invention, the titaniumdioxide may be plasma sprayed together with niobium oxide.

In a further aspect the present invention also provides a process forthe preparation of sub-stoichiometric titanium dioxide, TiO_(x), where xis below 2 having an electrical resistivity of less than 0.1 ohm.cm,which process comprises subjecting titanium dioxide to a plasmatreatment in an atmosphere which is oxygen deficient and which does notcontain oxygen-containing compounds. In carrying out this process thetitanium dioxide is preferably sprayed through a plasma flame, forexample a plasma flame using a mixture of argon and hydrogen as theplasma gas. Preferably the plasma flame will operate at a hightemperature to raise the temperature of the particles to above 2000° C.

The sputtering targets which are produced according to the process ofthe invention have a high electrical conductivity and thus are able torun at high power levels using conventional D.C. power supplies, withoutthe need for expensive arc diverter systems, or D.C. switching powersupplies, or the Twin-Mag System where two targets are sequentially usedas anode and cathode with a mid-frequency power supply, or any specialrequirements of a gas control system. Using the targets producedaccording to the present invention, D.C. sputtering can be carried outat power levels of up to 100 Kw. The main consequence is that largetarget bases, e.g. rotatable 3.5 metres long and 150 mm in diameter canbe coated up to a typical coating thickness of 6 mm.

The targets produced by the process of the present invention do notsuffer from significant arcing problems because titanium dioxide has ahigher melting point than titanium metal for which so called “vapourarcing” is a problem due to the lower melting point of the metal. Evenif some arcing does occur for titanium dioxide there is littleaccompanying damage to the target.

The present invention will be further described with reference to thefollowing Examples.

EXAMPLE 1

A rotatable target, water cooled on the inside to 35° C., comprising atube of stainless steel of diameter 133 mm and length 800 mm was coatedto a thickness of from 4 to 10 mm with sub-stoichiometric titaniumdioxide, TiO_(x), where x is below 2 as hereinbefore described by plasmaspraying titanium dioxide (rutile) having a particle size of from 10 to40 μm onto the target using argon as the primary plasma gas and hydrogenas the secondary plasma gas. 72 litres (60% argon, 40% hydrogen) wereused. The power level was 45 kW (455 A, 96V).

EXAMPLE 2

A commercial white pigment consisting of titanium dioxide in the anatasecrystal form was used. This powder is stoichiometric and electricallyinsulating. The powder was mechanically agglomerated and compacted intoflakes, ground, sieved (70-100 μm) and sintered at 1300° C. in air. Thesintered body was then ground and sieved to a particle size of 10-40 μm.The particles were yellow stoichiometric, non-conductive, titaniumdioxide with a rutile crystalline structure.

A rotatable target comprising a backing tube of aluminium (2.50 m longand 133 mm diameter) was prepared by plasma spraying of the above rutilepowder using argon as the primary gas and hydrogen as the secondary gas.75 litres (40% argon, 60% hydrogen) were used. The power level was 50 kW(110V, 455 A). The plasma spraying was carried out under a nitrogenatmosphere.

The target was rotated at 100 rpm and the torch translation was 2.5m/min until a coating 4 mm thick was obtained. The inside of thealuminium tube was water cooled to a temperature of 35° C. The coatedtarget had a resistivity of 0.07 ohm.cm. The target was subsequentlytested at power levels of up to 100 kW and worked well in the sputteringequipment without significant arcing. The deposition of titanium dioxidewas six times higher than the rate from a titanium metal target inreactive sputtering.

EXAMPLE 3

Example 2 was repeated with a low pressure vacuum plasma operating at200 mBar using titanium dioxide in the anatase form having a particlesize in the range of from 1 to 10 μm. Using the low pressure plasma,powders with a smaller particle size can be used.

On spraying onto a target base under the conditions of Example 2 theanatase was converted into a sub-stoichiometric rutile form of titaniumdioxide. The coated target had a resistivity of 0.02 ohm.cm.

EXAMPLE 4

A mixture of niobium oxide (25 parts by weight) and titanium dioxide (75parts by weight) having a particle size of from 0.1 to 2 μm wasagglomerated and compacted, dried and sintered at 1300° C. in air. Thesintered body was then ground to a particle size of 10 to 40 μm.

The powder mixture was then plasma sprayed under the conditions given inExample 2 onto an aluminium backing tube to a coating thickness of 4 mm.The coated target had an electrical resistivity of 0.5 ohm.cm and thuscould be used as a D.C. sputtering target.

1-14. (canceled)
 15. A large rotatable sputtering target which comprisesa target based plasma coated under cooling with sub-stoichiometrictitanium dioxide, TiO_(x), where x is below 2, optionally together withniobium oxide,
 16. A sputtering target as claimed in claim 15 whereinthe target base is water cooled during the plasma spraying.
 17. Asputtering target as claimed in claim 15 wherein the plasma spraying iscarried out using argon as a plasma gas and hydrogen as a secondaryplasma gas.
 18. A sputtering target as claimed in claim 15 wherein thetarget base is titanium, stainless steel, aluminum or copper.
 19. Asputtering target as claimed in claim 15 wherein the titanium dioxidewhich is plasma sprayed has particle size in the range of from 1 to 60micrometers.
 20. A sputtering target as claimed in claim 15 wherein thetitanium dioxide is plasma sprayed together with Nb₂O₃.
 21. A sputteringtarget as claimed in claim 15 wherein the sub-stoichiometric titaniumdioxide, TiO_(x), has a value of x in the range of from 1.55 to 1.95.22. A sputtering target as claimed in claim 21, wherein the sputteringtarget has a length of from 800 mm to 3.5 meters and has a cylindricalshape.
 23. A sputtering target as claimed in claim 22 which is 2.5 meterin length.
 24. A sputtering target as claimed in claim 23 which has adiameter of 133 mm.